Not a Lee Speed, but my J. Burrow Farquharson in .375 2 1/2" Fl. has a 1:16" left-hand twist, 7 groove, Engield rifling.

It shoots a 265 gr. 0.97" RNFPGC water-quenched w/w bullet (45.0 gr. RL-15) to point of aim;
old 300 gr. Hornady RNSP about 4-5" low; and a
335 gr. cast bullet (old Lyman mould) about 12" low.
All at 50 yds. using the 50 yd. fixed leaf.